Method for depositing a fluorine-doped silica film

ABSTRACT

The invention concerns a method comprising evaporating silicon and/or SiO x , wherein said evaporating is further defined as occurring in the presenceof oxygen if silicon or SiO x  with x less than two is being evaporated, to form a silicon oxide film at the surface of a substrate and in bombarding said silicon film, while it is being formed, with a beam of positive ions derived from both a polyfluorocarbon compound and a rare gas. The invention is useful for producing low-index antiglare films.

This application is a national phase application under 35 U.S.C. § 371of PCT Application No. PCT/FR01/02505 filed Jul. 31, 2001, which claimspriority to French Application No. 00/10149 filed Aug. 1, 2000, thecontents of which are incorporated herein by reference in theirentirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates in a general manner to a process for depositing afluorine-doped silica film (SiO_(x)F_(y)) on a surface of a substrate,in particular of an ophthalmic lens.

2. Description of Related Art

Silica-based (SiO₂) thin films are widely used in optics and moreparticularly in the field of ophthalmic optics. Such silica-based thinfilms are used in particular in anti-glare coatings. These anti-glarecoatings are conventionally constituted of the multi-layered stacking ofinorganic materials. These multi-layered anti-glare stackings usuallycomprise one or more layer(s) having a low refractive index in thevisible spectral field. Conventionally, these layers of low refractiveindex are constituted of a silica-based thin film.

The deposition techniques for such silica-based thin films are verydiverse, but deposition by evaporation under vacuum is one of the mostcommonly used techniques. These SiO₂-based thin films possess verysatisfactory mechanical properties and refractive indices usually of theorder of 1.48 for a wavelength around 630 nm.

However, in order to be able to improve the optical performances of theanti-glare stacking on the one hand and to generate novel systems ofanti-glare stacking, it would be desirable to be able to lower therefractive index of this low index film while preserving itssatisfactory mechanical properties.

In order to resolve this technical problem, it has already been proposedto generate porous silica (SiO₂) films, i.e. in which air is imprisoned.

Unfortunately, as well as the complex manufacturing techniques employed,the films thus obtained possess unsatisfactory mechanical propertieswhich are inferior to those of a conventional silica thin film.

Moreover, the use of fluorine-doped silica thin films is known in othertechnical fields, in particular in the field of microelectronics.

The films are obtained by chemical deposition in the vapour phaseassisted by plasma on discs for semi-conductors.

This technique induces a heating of the substrate which is brought tohigh temperatures, incompatible with the treatment of ophthalmic organicglasses.

Furthermore, these layers pose stability problems. The patentapplication EP-0.957.017 gives an account of diffusion problems offluorine to the outside of the fluorine-doped silica film which leads toadhesion problems.

The deposition of a silica film has been proposed in order to preventthis diffusion without, however, giving complete satisfaction.

The article “Characteristics of SiO_(x)F_(y) Thin Films Prepared by IonBeam Assisted Deposition” by F. J. Lee and C. K. Hwangbo describes thinfilms of fluorine-doped silicon oxide (SiO_(x)F_(y)). The articledescribes in particular the deposition of thin films of SiO_(x)F_(y) ofa thickness of about 600 nm on glass and silica substrates. The basicvacuum pressure is 1.2×10⁻⁴ Pa and the temperature of the substrate isabout 150° C. The silica is evaporated by means of an electron beam inthe presence of oxygen in the chamber and the silicon oxide deposit isbombarded during its formation by a beam of polyfluorocarbonated ionsformed by means of an ion gun starting from CF₄ gas.

The thin SiO_(x)F_(y) films obtained have refractive indices varyingfrom 1.394 to 1.462 and can be used as optical films.

However, the SiO_(x)F_(y) films obtained by the process of the abovearticle have the disadvantage of taking up water with time and of havingan unstable refractive index which increases with time.

The object of the present invention is thus a process for the depositionon a surface of a substrate of a fluorine-doped silica film(SiO_(x)F_(y)) with a low refractive index, stable over time and havingmechanical properties at least comparable to the films of the prior art.

SUMMARY OF THE INVENTION

According to the invention, the process for the deposition on a surfaceof a substrate of a fluorine-doped silica film (SiO_(x)F_(y)) comprises:

a) the evaporation of silicon and/or silicon oxide;

b) the deposition of silicon and/or silicon oxide evaporated at thesurface of the substrate in order to form on the said substrate surfacea silicon oxide film; and

c) the bombardment, during its formation, of the silicon oxide film witha beam of positive ions derived from a polyfluorocarbonated compound ora mixture of polyfluorocarbonated compounds, the process beingcharacterized in that the silicon oxide film is also bombarded, duringits formation, by a beam of positive ions derived from a rare gas or amixture of rare gases.

As indicated above, the deposit of silicon oxide during step b) of theprocess of the invention is obtained by evaporating silicon and/or asilicon oxide.

A silicon oxide of formula SiOx with x<2 or SiO₂ may be used. When SiOxwith x<2 is used, it is necessary that the ambient medium containsoxygen O₂.

Of course, a SiOx/SiO₂ mixture may be used. SiO₂ silica is preferred inthe framework of the invention.

The polyfluorocarbonated compound may be a linear, branched or cyclicperfluorocarbonated compound, and is preferably linear or cyclic.

Among the linear perfluorocarbonated compounds, mention may be made ofCF₄, C₂F₆, C₃F₈ and C₄F₁₀; among the cyclic perfluorocarbonatedcompounds, mention may be made of C₃F₆ and C₄F₈; the preferred linearperfluorocarbonated compound is CF₄ and the preferred cyclic compoundC₄F₈.

A mixture of perfluorocarbonated compounds may also be used.

The polyfluorocarbonated compound may also be a hydrogenofluorocarbon,

preferably selected from CHF₃, CH₂F₂, C₂F₄H₂. The hydrogenofluorocarbonmay also be linear, branched or cyclic.

Naturally, a mixture of perfluorocarbonated and hydrogenofluorocarboncompounds may be used.

The rare gas is preferably selected from xenon, krypton and theirmixtures. The preferred rare gas is xenon.

During the deposition of the fluorine-doped silica layer, the substrateis usually at temperature lower than 150° C., preferably lower than orequal to 120° C. and better still varies from 30° C. to 100° C.

In a preferred embodiment of the invention, the temperature of thesubstrate varies from 50 to 90° C.

The fact that the deposit according to the invention can be performed ata relatively low temperature makes it possible to form thin films on alarge variety of substrates and in particular substrates made of organicglass, such as ophthalmic lenses made of organic glass.

Usually, the process of the invention is carried out in a vacuum chamberat a pressure of 10⁻² to 10⁻³ Pa. If necessary, oxygen gas can beintroduced into the vacuum chamber during the deposition of the film.

The fluorine-doped silicon oxide films of the invention usually have athickness of 10 to 500 nm, preferably from 80 to 200 nm, and the atomicfluorine content of the films is usually from 6 to 10%.

The silicone content is usually of the order of 30% atomic.

The fluorine-doped silicon oxide films obtained by the process of theinvention have a refractive index n≦1.48, and preferably from 1.42 to1.45 (for radiation of wavelength λ=632.8 nm at 25° C.).

DESCRIPTION OF THE DRAWINGS

The remainder of the description refers to the appended figures whichrepresent respectively:

FIG. 1, a schematic view of an appliance for carrying out the process ofthe invention; and

FIG. 2, a schematic plan view of the appliance of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

The device for depositing thin films shown in the FIGS. 1 and 2 assistedby an ion beam is a standard device. This device comprises a vacuumchamber, the first extremity 2 of which is connected to one or morevacuum pumps and the other opposite extremity comprises a door 3. A coldtrap 4 can be placed in the chamber close to the extremity 2 connectedto the vacuum pumps. Within chamber 1 is located an electron gun 5comprising a crucible 6 designed to contain the silica to be vaporised.The substrates A to be coated are arranged on a support close to aquartz micro-balance 9. If need be, provision may also be made for anoxygen gas supply to chamber 10. The pressure in the chamber can bemeasured by means of a hot cathode pressure gauge 8. The supply line 11of the ion gun 7 is connected to three feed drive devices for gasesmaking it possible to simultaneously or independently supply the ion gunwith gases of the desired nature and/or flow rates.

In the present case, the vacuum chamber is a Leybold Heraeus chambercapable of attaining a basic vacuum of 5×10⁻⁵ Pa, the ion gun is MARK IICommonwealth gun, and the electron gun is a Leybold ESV gun.

For the control feed device of gases of the ion gun, a BROOKS mass flowcontrol device is used for argon gas, itself controlled by the MARK IIcontrol device. For the feed of xenon and the polyfluorocarbonatedcompound, mass flow control devices are used such as the multigascontrol device MKS 647 B in which the nature and flow rate of the gasescan be programmed.

The deposition on the substrates of the fluorine-doped silica filmaccording to the invention can be carried out in the following manner:

The chamber 1 is placed under a vacuum of 2×10⁻³ Pa (measured by meansof the hot cathode pressure gauge 8). The ion gun 7 is primed with argongas, then CF₄ gas and xenon are introduced at selected flow rates andthe argon flux is interrupted. The silica particles (SiO₂) placed in thecrucible 6 are preheated by the electron beam gun. When oxygen gas isused, it is introduced in the chamber at a fixed flow rate.Simultaneously, the electron beam gun and the ion gun are equipped withan obturator, and the two obturators of the electron beam gun and theion gun are opened simultaneously. The thickness of the deposit isregulated by the quartz micro-balance 9 near to the sample substrates.When the desired thickness of the films is obtained, the two obturatorsare closed, the electron beam and ion guns are cut, the supply of thevarious gases is stopped, and the vacuum of the chamber is broken. Thesample substrates coated with the fluorine-doped silica film accordingto the invention are then recovered.

The following examples illustrate the present invention.

EXAMPLES

By operating as previously described, flat surface silicon samples havebeen coated with fluorine-doped silica films. The refractive index atthe wavelength λ=632.8 nm and at 25° C. of the fluorine-doped silicafilms formed was measured at different times after the formation of thefilms. The absorption of water by the films formed at different timesafter the preparation of the films was also determined by infraredspectrometry, this absorption being characteristic change of the filmwith time. The conditions for depositing the fluorine-doped silica filmsare indicated in Table I, while the properties of the films obtained, inparticular the refractive index and the detection of the presence ofwater by infrared spectrometry and the thickness of the layers obtained,are indicate in Table II.

TABLE I Deposition conditions Polyfluoro- carbonated Deposition Ion gunIon gun Polyfluoro- compound Chamber Substrate rate anode anodecarbonated flow rate Xe flow rate O₂ flow rate pressure temperatureExample N° (nm/s) current (A) voltage (V) compound (cm³/minute)(cm³/minute) (cm³/minute) (Pa)⁽¹⁾ (° C.) Comparative 0.51 0.53 160 CF₄2.3 — —   4.10⁻³ 70° C.⁽²⁾ C1 Comparative 0.18 0.3 100 CF₄ 1.8 — —5.3.10⁻³ 180° C.⁽³⁾  C2 1 0.75 4 150 CF₄ 2.5 2.9 4 1.8.10⁻² 70° C.⁽²⁾ 20.75 0.5 100 CF₄ 1.5 0.5 4 7.9.10⁻³ 70° C.⁽²⁾ 3 0.5 4 150 C₄F₈ 1 2.7 152.4.10⁻² 70° C.⁽²⁾ ⁽¹⁾Measured during deposition ⁽²⁾Temperature obtainedby heating the substrate by means of the ion gun ⁽³⁾Temperaturemaintained throughout deposition by a heating device.

TABLE II Properties of the SiOxFy films Refractive index at λ = 632.8 nmPresence of water (IR) Example Thickness After 1 After 24 After 2 After2 After 2 After 1 After 2 After 2 N° (nm) hour hours days weeks monthshour days weeks Comparative 125 1,415 — 1,465 — — No Yes — C1Comparative 110 1,400 1,448 — — — — — — C2 1 190 1,429 — — 1,432 1,435No No No 2 180 1,444 — — 1,449 1,450 No No No 3 190 1,434 — — 1,437 — NoNo No

The results of Table II show that the bombardment with an ion beamderived simultaneously from a polyfluorocarbonated compound and a raregas, in this case xenon, makes it possible to obtain a particularlynoteworthy stabilization of the refractive index with time. In fact, therefractive index of the films of the comparative examples C1, C2increased by 3.5% after two days and by 3.4% after 24 hours,respectively, whereas the refractive index of the films of the examples1 to 3 obtained by the process of the invention show only an increase ofless than 0.35% after two weeks, and less than 0.42% after 2 months.

What is claimed is:
 1. A process for depositing on a surface of asubstrate a fluorine-doped silica film (SiOxFy) comprising: evaporatingsilicon and/or SiO_(x), wherein said evaporating is further defined asoccurring in the presence of oxygen if silicon or SiO_(x) with x lessthan two is being evaporated; depositing the evaporated silicon and/orSiO_(x) onto a surface of a substrate in order to form a silicon oxidefilm on the substrate; bombarding the silicon oxide film, duringformation, with a beam of positive ions derived from apolyfluorocarbonated compound or of a mixture of polyfluorocarbonatedcompounds; and bombarding the silicon oxide film, during formation, by abeam of positive ions derived from a rare gas or a mixture of raregases.
 2. The process of claim 1, wherein the polyfluorocarbonatedcompound is a linear or cyclic perfluorocarbonated compound.
 3. Theprocess of claim 2, wherein the linear perfluorocarbonated compound isCF₄, C₂F₆, or C₃F₈ and the cyclic perfluorocarbonated compounds is C₃F₆or C₄F₈.
 4. The process of claim 3, wherein the cyclicperfluorocarbonated compound is C₄F₈.
 5. The process of claim 1, whereinthe polyfluorocarbonated compound is a hydrogenofluorocarbon.
 6. Theprocess of claim 5, wherein the hydrogenofluorocarbon is CHF₃, CH₂F₂, orC₂F₄H₂.
 7. The process of claim 1, wherein the rare gas is xenon orkrypton.
 8. The process of claim 7, wherein the rare gas is xenon. 9.The process of claim 1, wherein during the depositing of the siliconoxide and the bombarding, the substrate is at a temperature lower than150° C.
 10. The process of claim 9, wherein the substrate is attemperature lower than 120° C.
 11. The process of claim 9, wherein thesubstrate is at a temperature between 30° C. and 100° C.
 12. The processof claim 9, wherein the substrate is at a temperature between 50° C. and90° C.
 13. The process of claim 1, further defined as carried out in avacuum chamber at a pressure of 10⁻² to 10⁻³ Pa.
 14. The process ofclaim 1, further defined as carried out in a chamber into which oxygengas is introduced during the deposition of the evaporated silicon and/orsilicon oxide into the surface of the substrate.
 15. The process ofclaim 1, wherein the fluorine-doped silicon oxide film formed has athickness of 10 to 500 nm.
 16. The process of claim 15, wherein thefluorine-doped silicon oxide film formed has a thickness of 80 to 200nm.
 17. The process of claim 1, wherein the atomic fluorine content ofthe fluorine-doped silicon oxide film is from 6% to 10%.
 18. The processof claim 1, wherein the silicon oxide film has a refractive index, n, ofless than 1.48 at a wavelength of 632.8 and at 25° C.
 19. The process ofclaim 18, wherein the refractive index, n, is from 1.42 to 1.45 at awavelength of 632.8 nm and at 25° C.
 20. The process of claim 1, whereinthe substrate is as an ophthalmic lens.
 21. The process of claim 1,wherein the substrate is as a flat silicon sample.